This invention relates to the coating of paper with latex-based coating compositions. More particularly, it relates to a process for obtaining a brighter and more opaque coated paper and to paper coated in this way.
In order to provide good printing surfaces, it is normal to coat paper with aqueous-based compositions which have been formulated for this purpose. Among the compositions which have been used are coatings comprised essentially of a major proportion of a mineral or organic pigment and a minor proportion of a binder in the form of a latex of a film-forming polymer. Suitable pigments have included finely divided clay, calcium sulfoaluminate also known as satin white, oxides of titanium, aluminum, silicon and zinc, calcium carbonate and microsized particles of high softening point polymers which are insoluble in the binder. Suitable binder polymers have been those which are film-forming at ambient and higher temperatures. The coating is spread over the paper surface by a roll coater, trailing blade, air knife, brush or other known means, after which it is dried.
The method of drying the coated paper has generally involved heating it to a sufficiently high temperature to evaporate the water and cause coalescence of the polymeric binder particles. The particles of the binder polymer will coalesce when they are dried above the minimum film-forming temperature (MFT) of the polymer. Heating can be carried out by passing the coated paper through a hot air circulating oven or by contacting it with the surfaces of heated rolls or both. It is also known to dry the coating at a temperature below the minimum film-forming temperature of the binder particles to avoid coalescence of these particles and then subjecting the dried coating to a hot calendering treatment to cause coalescence of the particles and produce a glossy surface on the paper. For more details regarding the foregoing procedures see U.S. Pat. Nos. 3,399,080 and 3,873,345 and TAPPI (Technical Association of the Pulp and Paper Industry) Monographs 7, 9, 20, 22, 25, 26, 28 and 37. While coatings of acceptable opacity and brightness can be obtained by these known procedures, it is desirable to obtain coatings in which these and other properties are enhanced. For example, improvement in ink receptivity and gloss are also an ever present goal in the industry.